1989 C126 420 SEC crank position sensor faulty?

[HowardP]

This thread supercedes my recent one about testing the fuel pump relay, because my car has suddenly decided not to start - not even a cough to show it's trying.

I've established there is a spark at the plugs and that the fuel pump is working, which basically leaves the crank position sensor which, if I understand correctly, governs the timing.

The way I see it therefore, is that I have a spark, and I have fuel being delivered, just not necessarily at the right time.

Apparently a faulty CPS can display a number of symptoms, the most extreme of which is the engine failing to start, although I understand the likelyhood of this happening suddenly is rare.

Am I on the right track? Is there any way of testing the CPS in situ? I notice it is connected to a circular 9 pin diagnostic socket on the front left inner wing, which might, or might not, be relevant.

Has anyone replaced one of these on this engine, because it looks a real pig to do, and any helpful tips would be appreciated?

 

[alabbasi]

I read your other post on the FPR and am not sure if you have the pins correct. I think it's the two (top and bottom) in the middle but it's been a while since I jumped the FPR on a W126. You can use a paper clip to do it without the relay in place (watch it as it gets hot) and hear the fuel pump running.
You can also under the 17mm fitting for the inlet hose and see if you have pressure.

Alternatively, pour some fuel into the intake (just a shot glass full) and see if it will fire. If it does , then your ignition system is probably fine.

Have you checked the cap and rotor?

AFAIK, the crank position sensor is NLA so I would check everything else before I decide that the CPS is bad.

 

[merc85]

If the igition system, is not fine have you thought about the Ignition amplifier? my Dad had this play up on his c126 500

 

[ray_hennig]

Just a quicky here but, if you have a spark, I think your crank position sensor and ignition controller are working.

Anyone else agree?

RayH

 

[HowardP]

When my son and I tested the spark by holding the spark plug against the engine my son thought the spark looked a bit weak, but possibly the rusty manifold bolt we used was a contributory factor! Perhaps we will try again tonight, although we have no way of knowing if the spark is strong enough, other than by a visual check.

I suppose it's possible the coil is faulty, thus not providing enough power to the distributor.

As far as the fuel is concerned, there is a smell of fuel if the engine is cranked for a few seconds, so on that basis we assumed that fuel was getting delivered. The fuel pump can be heard when the ignition is switched on, so I will disregard the FPR for the moment. As you say, it would be a good idea to crack open the feed pipe to the fuel distributor to make sure there is pressure. I'll also try your tip about pouring a little fuel into the air intake.

Thinking about the CPS, I'm now not sure what contribution it makes to the running of the engine. On modern engine it is relevant to timing, but my car has a mechanical distributor, so what does it do, exactly, I wonder. It's obviously critical in some way, because a faulty one can cause all sorts of problems.

The distributor cap and rotor arm are both brand new, as are the spark plugs.

 

[HowardP]

The thing that really puzzles me though is why it happened without any warning. From what I've experienced, and read, electrical components usually start playing up before they fail completely. On the other hand, mechanical components can fail with no warning at all.

As I mentioned in my other thread, it started, idled and ran just fine after replacing various components, with no surging or low idle issues. I ran the car for about 30 minutes before putting it back in the garage - and that's when the current problem appeared.

 

[HowardP]

I've just spoken to my local M-B dealer and there are TWO "sender units". The front one is A0021533428 at £258 and the rear is A0021534628 at £39. Also available elsewhere, so not NLA - yet!

Perhaps it's time to consider calling out a mobile mechanic who can carry out some tests with the proper equipment, preferably one who is used to cars from the 80s and 90s.

 

[grober]

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1988/420SEL/WIRING DIAGRAMS/3168.pdf

In this setup at least the FPR gets its engine running signal from the tachometer

EZL explained
https://www.benzworld.org/forums/at...1-w126-420sel-no-spark-issue-ezl-ignition.pdf

these pre digital electronics engines are best diagnosed by the older oscilloscope based systems like the Sun engine analysers where HT firing lines can be displayed

 

[optimusprime]

howerd dont spend any cash at the moment ... but if you need a starting place it would be the coil or coils what engine do you have ?

 

[Will]

Going off the diagram kindly posted by Grober above, as per your other thread it’s pins 7 & 8 for the fuel pumps. Which corresponds to 7 and X on your socket, or 87 and X on the relay.

Could you post a picture of the underneath of your relay, or if not the part number?

Just because you can hear the pumps prime (and then stop) it does not mean they are delivering fuel when cranking - hence the suggestion to bridge the relay so they run continuously.

What do the plugs look/smell like after cranking - are they damp/smell strongly of fuel?

 

[optimusprime]

Would this set up have 2 coils not one? v8 yes ?

 

[grober]

Would this set up have 2 coils not one? v8 yes ?

Not at this point in time don't think .
91 W126 420SEL No Spark Issue - Mercedes-Benz Forum
Later V8 systems had two iirc
K6JRF Auto Page

 

[HowardP]

Graeme, thanks for the links. The first thing I noticed after a quick glance is that it is the rear sensor that is part of the ignition circuit, not the front one, which is not what I was expecting, but then again the fact that the front one is connected to the diagnostic socket should have alerted me. Anyway, I'll study the information over a cup of coffee!

Will, I have attached pictures of my relay and its housing, which I hope helps.



@optimusprime, there is only one coil.

I will try the suggestions about checking fuel delivery, the state of the plugs, and maybe even pluck up courage to pour neat fuel down the intake.

 

[grober]

the relay with that part no is listed as an air con relay.

Pinouts of Klima relay - PeachParts Mercedes-Benz Forum

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[alabbasi]

I'm now not sure what contribution it makes to the running of the engine. On modern engine it is relevant to timing, but my car has a mechanical distributor, so what does it do, exactly, I wonder. It's obviously critical in some way, because a faulty one can cause all sorts of problems.

On CIS E cars , you can't adjust the distributor to adjust timing. It's all managed using the CPS and EZL.

Easiest thing to do is squirt fuel into the intake and see what happens. If it runs for a bit. It's a fuel issue.
Check cap and rotor, I know you said it's new but check anyway. It takes 30 seconds. Also check that wires are installed correctly. Firing order is on drivers side (in your case) cam cover.

 

[HowardP]

Firstly, I owe everyone an apology. The (unmarked) FPR is directly behind the A/C relay, which was a schoolboy error, so apologies to everyone for wasting your time on that wild goose chase. I can now bridge 7 & 8, just as part of the process of elimination.

Thanks to the very helpful link that @grober sent I now see that the distributor is just a non-adjustable slave unit, dictated to by the EZL. The EZL in turn is reliant on information from various sources, the most critical presumably being the CPS, which has remained untouched since I bought the car, as has the EZL.

In the information about the EZL there is the following phrase:

"If the EZL does not receive a signal from the position sensor, the complete ignition system is inoperable".

Apparently here should be a resistance of between 680 and 1200 Ohm between two particular sectors of the sensor face, which means I can at least test it with a multimeter. All I have to do is remove it! However, I'll double check everything again, and try throwing a little fuel down the throat of the beast.

Al, the firing order is fine because I marked the leads before removal, and it ran perfectly in the morning, since when the leads have not been removed, and the cap will only fit on one way.

 

[optimusprime]

As grober said in post 8 about the signal from the revcounter . And if the rev counter fails then it to will interupt the start up sequence . So keep in mind,, after you have tried all help given in posts ., and if you dont solve this,then remove the wire from revcounter and try again.

 

[alabbasi]

Firstly, I owe everyone an apology. The (unmarked) FPR is directly behind the A/C relay, which was a schoolboy error, so apologies to everyone for wasting your time on that wild goose chase. I can now bridge 7 & 8, just as part of the process of elimination.

Thanks to the very helpful link that @grober sent I now see that the distributor is just a non-adjustable slave unit, dictated to by the EZL. The EZL in turn is reliant on information from various sources, the most critical presumably being the CPS, which has remained untouched since I bought the car, as has the EZL.

In the information about the EZL there is the following phrase:

"If the EZL does not receive a signal from the position sensor, the complete ignition system is inoperable".

Apparently here should be a resistance of between 680 and 1200 Ohm between two particular sectors of the sensor face, which means I can at least test it with a multimeter. All I have to do is remove it! However, I'll double check everything again, and try throwing a little fuel down the throat of the beast.

Al, the firing order is fine because I marked the leads before removal, and it ran perfectly in the morning, since when the leads have not been removed, and the cap will only fit on one way.

I understand, but it pays to double check as you did with the FPR . we're all providing suggestions based on past experience and it's a simple system. There's not much else that can fail.

 

[optimusprime]

The way to go is check spark , then check fuel pressure . If the cps is no good the computer wont give information to the ezl to fire up . If ezl is no good no spark what so ever . Faulty coil - poor spark .Could be a hundred things . Rule out CPS by spinning over the engine and look for a movement on the rev counter.

 

[HowardP]

I have to say this is a steep learning curve! Anyway, I shall bear in mind the comments about the rev counter, and will triple check everthing as well, just to be sure.